1. Field of the Invention
The present invention relates to a solid image pickup apparatus for transferring a signal charge photoelectrically converted by a photoelectric converter such as a photodiode, particularly to a technique for preventing impact ionization to enhance an image quality.
2. Related Background Art
Examples of a typical solid image pickup apparatus include a CCD image sensor. The CCD image sensor is easily integrated, and therefore broadly used in a scanner, digital copying machine, video camera, and the like in which a large number of pixels are required.
FIG. 7 is a plan view showing a schematic configuration of the CCD image sensor. The CCD age sensor of FIG. 7 includes: a photoelectric converter 11 in which a plurality of photosensitive pixels 10 are linearly arranged, a CCD register 13 for transferring a signal charge photoelectrically converted by the photoelectric converter 11 and subsequently sent via a shift electrode 12 in a direction of arrows in FIG. 7, and an output circuit 14 for outputting an analog signal in accordance with the signal charge moved to the end of the CCD register 13.
In recent years, with an increase of speed of an electronic apparatus, there has been an increasing demand for an increase of an output data rate of the CCD image sensor. Because of this, the speed is raised by shortening a gate length of MOSFET in the output circuit 14, enhancing a driving ability of MOSFET, or increasing a current flowing through the circuit.
However, when the gate length of the MOSFET is shortened, an electric field between source and drain is heightened, and an electron/hole pair is generated by impact ionizations. It is known that the electron/hole pair is recombined to emit light as shown it FIG. 8 (authored by Takeda, Nikkei MacGrow Hill Co., “Hot Carrier Effect”, p 37, 1987).
Additionally, in FIG. 8, reference numeral 30 denotes a gate electrode, 31 denotes a source region, and 32 denotes a drain region.
When the light reaches the photoelectric converter 11 of FIG. 7, the signal charge attributed to the light is generated by the photoelectric converter 11, and this deteriorates an image quality.